Meaning of “fester” - Learner’s Dictionary

fester

verb [ I ] uk us /ˈfestər/
FEELING

If a bad feeling or situation festers, it becomes worse over a period of time:

Hatred between the two groups has festered for years.
INJURY

If an injury festers, it becomes infected:

a festering wound

(Definition of “fester” from the Cambridge Learner’s Dictionary © Cambridge University Press)